package com.collection;

import java.util.HashSet;
import java.util.TreeSet;

public class TreeSetTest {
	public static void main(String[] args) {
		TreeSet treeSet = new TreeSet();
		//no compile error, but get execption when run it
		treeSet.add(null);
		
		//can contain a null value
		HashSet hs = new HashSet();
		hs.add(null);
	}
}
